Quote:

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It is a far stronger Mechanism than the Top Lever, and it admits of nearly ten times as much pressure being applied to withdraw the bolt as can be got from a Top Lever

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The above quote is the most important feature of the Jeffery snap action 600NE IMO.
Because the 600, (even when developing normal acceptable pressures) develops an extremely high thrust against the breech face, it can benefit from having an extremely strong system to withdraw the bolting, especially if pressure is higher than usual and causes some springing of the action.
I don't know if that particular action had any self or assisted opening.
If it did, I think that would have been a secondary feature, the primary feature was having a powerfull leverage over the underbolting.
A system with more leverage than what the top lever can provide.

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